1. Field of the Invention
The present invention relates to a manufacturing method of a recycled plate for a sliding nozzle, the method including performing processing for extending an endurance period on a plate used for the sliding nozzle that performs discharge control and flow control of molten steel in a steel manufacturing process.
2. Description of the Related Art
A sliding nozzle is a device that is used for discharge control and flow control of molten steel when discharging the molten steel from a molten steel vessel such as a ladle and a tundish in a steel manufacturing process. As illustrated in FIG. 8, a common sliding nozzle 110 includes two plates of a fixed plate 111 and a movable plate 112 that are formed by refractory bricks. The fixed plate 111 is fixed to the bottom portion of an upper nozzle 101 fitted into an opening of a bottom portion 90 of the molten steel vessel. The movable plate 112 is a plate that reciprocates or rotates while sliding relative to the fixed plate 111. A lower nozzle 102 is fixed to the movable plate 112. Holes 111h and 112h penetrate through the fixed plate 111 and the movable plate 112, respectively. The position of the hole 111h of the fixed plate 111 is identical to an opening 101p of the upper nozzle 101 and the position of the hole 112h of the movable plate 112 is identical to an opening 102p of the lower nozzle 102. With this configuration, if the movable plate 112 slides relative to the fixed plate 111, the opening 101p of the upper nozzle 101 and the opening 102p of the lower nozzle 102 are switched to be communicated or non-communicated with each other. This controls discharge or stop of the discharge of the molten steel from the molten steel vessel. Furthermore, the size of a space through which the opening 111h of the fixed plate 111 and the opening 112h of the movable plate 112 are communicated is changed so as to perform flow control of the molten steel.
Although the sliding nozzle 110 including two plates (the fixed plate 111 and the movable plate 112) is illustrated in FIG. 8, a sliding nozzle 120 including three plates of an upper fixed plate 121, a lower fixed plate 122, and a movable plate 123 has been used conventionally, as illustrated in FIG. 9. The upper fixed plate 121 is fixed to the upper nozzle 101 fixed to the bottom portion 90 of the molten steel vessel. The lower fixed plate 122 is unmovable and the lower nozzle 102 is attached to the lower fixed plate 122. The movable plate 123 slides between the upper fixed plate 121 and the lower fixed plate 122.
The plates used for the sliding nozzle as described above are severely damaged with use. For example, portions of the plates that make contact with the molten steel are eroded physically by the flowing molten steel. Furthermore, the temperature of the molten steel is extremely high, so that cracking is occurred on the plates due to thermal shock. The cracking is also occurred by structural spalling due to penetration of the molten steel into matrices of the plates. In addition, contents of a refractory material constituting the plates are lost because of elution into the molten steel or gasification, so that the matrices become porous. Moreover, the plates slide relatively, so that contact surfaces thereof are worn and the surface roughness is increased.
The endurance periods of the plates for the sliding nozzle used under such a severe environment are short and the plates need to be exchanged by brand-new plates, frequently. This arises a problem that the cost is increased. Furthermore, in order to avoid a significant trouble such as leakage of the molten steel, the plates are exchanged by the brand-new plates earlier than the usage limits thereof, resulting in waste of resources.
In order to solve the above problems, the present applicant has proposed a recycling method of performing processing for extending an endurance period on a used plate in the past (see, Japanese Patent No. 4980275). The recycling method is a method of impregnating a refractory layer of the used plate with tar or pitch, and then, heating it so as to remove a volatile component from the tar or the pitch. With this method, the tar or the pitch penetrates into cracking, recesses on the surface having the increased roughness, and increased opening pores that are occurred on the refractory layer of the plate with use so as to densify the refractory layer. Further, extension of the cracking is suppressed with an adhesiveness of the tar or the pitch. Accordingly, the above-mentioned recycling method is capable of extending the endurance period of the used plate. Moreover, the recycled plate can be distributed at lower cost than a brand-new product. This makes it possible to reduce the cost taken for maintaining the sliding nozzle and use resources effectively.
However, with the above-mentioned conventional method, when the recesses such as the cracking occurred on the plate with use are minute, the recesses can be repaired with the impregnation of the tar or the pitch but the recesses having large sizes at some degree cannot be repaired. For this reason, the used plate on which even less recesses having the large sizes have been occurred cannot be recycled with the above-mentioned method. In consideration of this, a technique that makes it possible to recycle even a plate on which recesses having large sizes have been occurred and widen a range of a used plate as a target for recycling has been required.